The present invention relates to the activation of a normally nonconductive surface to the electroless plating.
Electroless plating involves the spontaneous chemical reduction and deposition of a metal onto a normally non-conductive surface, such as a polymeric surface activated by provided free noble metal sites. The deposited metal coating acts as a buss to allow a thicker metal coating to be built up electrolytically.
On a commercial scale, an electroless plating operation consists of a number of batch-wise stages in series through which articles to be electrolessly plated are passed with one or more rinse operations occurring between each stage to minimize "drag in" of chemicals from one stage into the next. Although residence time will vary from stage to stage, the cycle time for transport apparatus require a degree of coordination of residence times as between the several stages involved. This is necessary to make the operation economical without sacrificing the quality of the electrolessly deposited plate.
The most common substrates plated are articles molded from organic polymers such as acrylonitrile-butadiene-styrene resins. For organic polymers, the first stage typically involves an organic or mechanical deglazing of the article surface. This is followed by etching in a strong oxidizing acid, such as chromic acid or mixtures of chromic and sulfuric acid. The most uniform acid etch is that described in our U.S. Pat. No. 3,366,130 issued June 6, 1972.
The function of the etch operations are to form minute sites for the deposition of a noble metal without destroying the integrity of the polymeric surface.
As indicated, between each stage there is performed one or more rinse operations. They are employed to remove from the articles to be plated, as well as the racks on which they are typically contained, any material "dragged out" from a previous bath to prevent "drag in" into a subsequent bath. "Drag in" can cause contamination of the subsequent bath or, in some instances, decomposition of the subsequent bath. Decomposition can occur in the surface activation bath and/or the electroless plating bath.
As indicated, surface activation involves the formation of noble metal sites on the surface of the article to be electrolessly plated. The noble metal most typically used is palladium.
The prior methods of depositing a noble metal involve two distinct stages aside from any rinse operations which may be employed. One method involves contacting the article with a sensitizing solution containing stannous chloride, followed by immersion in an activator solution such as palladium chloride where Pd.sup.+.sup.+ is reduced to Pd.degree. on the surface of the article.
While useful, the greatest disadvantage of this method of activating a surface for electroless plating is the phenomenon known as "skip plating". Skip plating is the result of unequal absorption or reduction leaving inactive areas or areas less active than others at which an electroless plate will not be deposited. Because of "skip plating", article rejection or recycle rate is high.
Another employs a highly acidic solution of a noble metal colloid, typically a palladium colloid, maintained in suspension by a protective colloid, i.e. stannic acid colloids. Activating solutions of this nature are described in U.S. Pat. No. 3,011,920 to Charles R. Shipley, Jr.
In their preparation, a stannous salt, such as stannous chloride is added in an amount greatly in excess of that required to reduce Pd.sup.+.sup.+ to Pd.degree. to contemporaneously form the protective colloid.
Colloidal palladium formed by other reductants and protected by other protective colloids under alkaline conditions are also mentioned. They are, however, stated to be less effective than the acidic colloids and, as presently known, have not been used to any extent on a commercial scale.
Because the colloidal palladium as well as the protective colloid are co-absorbed by the substrate to be electrolessly plated, the article is immersed in an acidic or alkaline accelerator solution to remove the protective colloid and expose the absorbed noble metal.
The acceleration step can be eliminated if the electroless plating solution is highly alkaline in nature and capable of removing the protective colloid. This route, however, is not commercially feasible since prolonged immersion times, i.e. 15-20 minutes, are required before electroless plating begins. This method, therefore, also requires, in substance, two stage activation.
While the use of an activator solution based on a colloidal noble metal suspended by a protective metal represents a significant improvement over the two stage activation operation previously discussed, it also has several drawbacks.
To prepare a functional solution involves a number of complex and time consuming chemical reactions. These chemical reactions are evidenced by a number of color changes, and may take several weeks to go to completion. Heating to temperatures as high as boiling for a period of time can accelerate the reactions.
The resultant activator solutions are also sensitive to both temperature and pH. Elevated temperature can result in decomposition of the bath with attendant precipitation of the noble metal. Because of this, a "growth factor" must be coped with to account for the "drag in" of materials from a previous bath or rinse which tends to dilute the bath. As a result, as chemicals are consumed, a portion of the bath must be periodically discarded and makeup chemicals added over and above the amount of chemicals utilized in activating the surfaces of substrates for electroless plating.
Independent of this, the use of an activator solution of this nature also requires the loss of a considerable quantity of the metal employed as the protective colloid. When colloidal palladium is protected by a stannic acid colloid, for instance, the excess tin required for the protective colloid is lost in the accelerator solution and eventually discarded as waste.